I have used the standard aluminum Coca-cola can as similar shim material in the past. Just be sure that when you insert the shim that nothing is sticking out that can rub the sideplate or frame.
I mhave made these shims by cutting a piece just short of the width of the slot in the trigger, and then bent 90 degrees and trimmed to fit between the trigger bar and the back of the slot. The trigger bar keeps the shim from slipping out of place when properly assembled. Take ot that this is a kludge, but it works well, and makes swapping parts out of a matched shooter unnecessary at the cost of a can of Coke, and a few minutes with a tin snips, and some files or sandpaper (for use on the shim only!)